The invention relates to a pressure exchanger for transfer of pressure energy from one fluid flow to another, wherein the pressure exchanger comprises a housing with an inlet and an outlet duct for each fluid flow, a rotor which is arranged for rotation about its longitudinal axis in the housing, and which has at least one through-going duct, which extends from one end of the rotor to the other end, considered in the axial direction, and alternately connects the inlet duct and the outlet duct for one fluid with the outlet duct and the inlet duct respectively of the other fluid and vice versa during the rotation of the rotor.